1. Field of the Invention
The present invention relates to imaging lens units for solid-state image sensors which are used in small imaging devices used in small low-profile electronic devices such as mobile terminals including mobile phones and hand-held games and PDAs (Personal Digital Assistance).
2. Description of the Related Art
With the recent expansion of the market for mobile terminals with imaging devices, the use of a high-resolution small solid-state image sensor in an imaging device has been spreading.
With the growing tendency for smaller and higher resolution image sensors, imaging lens units are required to provide higher performance in terms of resolution and image quality. Also, with the spread of such lens units, there has been demand for less costly imaging lens units.
An imaging lens unit including more than one lens has been widely used in order to meet the demand for higher performance and there has been proposed an imaging lens unit including five lenses which is more suitable for performance enhancement than an imaging lens unit including three or four lenses.
For example, JP-A No. 2007-264180 (Patent Document 1) discloses an imaging lens unit intended to provide higher performance, which includes the following constituent lenses arranged from the object side in the following order: a first lens with positive refractive power whose object side surface is convex, a second lens as a meniscus lens with negative refractive power whose concave surface faces the image side, a third lens as a meniscus lens with positive refractive power whose convex surface faces the image side, a fourth lens with negative refractive power whose surfaces are both aspheric with its image side surface concaved near the optical axis, and a fifth lens with positive or negative refractive power whose surfaces are both aspheric.
JP-A No. 2007-298572 (Patent Document 2) discloses an imaging lens unit intended to provide higher performance, which includes the following components arranged from the object side in the following order: an aperture stop, a first lens with positive refractive power, a second lens with negative refractive power bonded to the first lens, a third lens as a meniscus lens with its concave surface facing the object side, a fourth lens as a meniscus lens with its concave surface facing the object side, and a fifth lens as a meniscus lens having at least one aspheric surface with its convex surface facing the object side.
For the imaging lens units described in Patent Documents 1 and 2, which use five lenses for performance enhancement, it is difficult to achieve both miniaturization and adequate aberration correction because the total lens system length is large. In addition, since they use a glass material, it is not easy to reduce the cost.